Preparation of silicate phosphors, including firing in the presence of steam



Patented Oct. 10, 1950 PREPARATION OF, SILICATE PHOSPHORS, INCLUDING FIRING IN THE PRESENCE OF STEAM Herman C. Froelich, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Application December 6, 1946, Serial N 0. 714,672

8 Claims. (01. 252-3015) My invention relates generally to phosphors, that is, fluorescent or phosphorescent materials which are rendered luminescent by impinging radiations such as ultra-violet rays, cathode rays, X-rays, etc. More particularly, the invention relates to the preparation of thermally synthesized inorganic phosphor compositions which have, as matrix, a silicate compound. An object of the invention is to provide a process of prewhere the'crystal structure of air and steam fired silicates seems'to be identical, yet the brightness of steam fired silicate may be several fold that of air fired phosphor. Another example, involving an activator other than manganese, is cerium- 50 activated aluminum silicate phosphor. Batches of this phosphor have turned out twice as bright in mixtures of steam with nitrogen or hydrogen, compared with dry nitrogen or hydrogen alone. Still other examples are titanium-activated silicates, particularly those of metals of group II of the periodic system of atomic weight below 200, such as calcium', wherein the phosphor has been improved a third in output by firing insteam.

paring such phosphors which results in increased Since chemical effects could not be associated efl'lciency of the phosphor or a substantial reducwith the exposure to steam of such compounds as tion in the time of firing required to form the CaSiOs or MgSiO3, the function of the steam is phosphor, or both. apparently catalytic. This would indicate that This application is a continuation-in-part of the arrange in Space of ll the atoms and my applications Serial No. 498,405, filed August ions in a phosphor crystal is brought about more 12, 1943, now abandoned, No. 488,956, filed May sp di y and m perfectly when the sp 29, 1943-, now abandoned, and No. 499,642, filed is fired in steam instead of in air. August 23, 1943'," now Patent No. 2,415,129. In other cases steam has a chemical function The synthesis of phosphors is usually carried as well, particularly when it is used in conjuncout by firing the ingredients at elevated temperation with hydrogen. In the preparation of lowtures of the order of 700-1300 C. in air. In spetemperature yellow-fiuorescing manganese-acticial instances, a reducing atmosphere has been vated zinc silicate, for example, its presence employed to prevent oxidation of an activator to serves tokeep the reduction of ZnO to a minia higher valent form. However, according to the mum, according to the reversible equation: present invention, I have discovered that steam ZnO+H2ZZn+H2Q Considering the law of is a most effective catalyst for reactions involving mass action, it will be appreciated that large silicate phosphors, the formation of the phosamounts of H20, and only large amounts, will phors being accelerated and their brightness in help prevent a large loss of zinc as metal'vapor. many cases being improved several fold if they This combination of chemical and kinetic funcare fired in steam instead of air. 39 tion of the steam has made it possible to produce The action of the steam seems to be two-fold, yellow zinc silicate phosphor of greater brightthat is, kinetic and chemical. Whensteam func ness, greater ZnO:SiO2 mol ratio, higher MnO tions kinetically, it catalyzes the formation of content, higher percentage of long-wave emisthe phosphors, and produces brighter phosphors. sion, and greater particle fineness, than by any It is not known just what atomic mechanism may 35 other known method. be responsible for these effects. As far as I have One of the chief dilficulties in the preparation been able to determine, no changes in the crystal of manganese-activated phosphors is the incorstructure of the phosphor'an'd no changes in the poration of the manganese as divalent Mn ion valence of the activators are involved. into the phosphor matrix. When manganese Thus, calcium silicate phosphor containing a 40 compounds such as the. lower oxide MnO are small percentage of manganese oxide (MnO) has heated in air, they oxidize to form higher valent the same crystal structure whether fired in steam compounds such as MnzOs. The latter are not or in air, but it forms a much brighter phosphor soluble in most phosphor matrices. The prepawhen fired in steam. Manganese-activated magration of manganese-activated silicates, for exnesium silicate phosphor is another example ample, involves first an oxidation of divalent MnO to trivalent M11203, then a dissociation (or reduction) of the latter to give MnO and oxygen, and finally the solution of MnO throughout the matrix lattice. In my Patent 2,408,621 and'the divisional application thereof, Serial No. 666,776,

filed May 2, 1946, I have described means of effecting this reduction and re-solution of MnO, such as treatment with hydrogen or heating of the finished phosphor in vacuo. Steam seems to be also a good catalyst for the dissociation of M11203, and MnO may be formed even though no actual chemical reduction is involved as when hydrogen is used.

I will now set forth further particulars regardin preferred processes of preparing the various phosphors referred to above.

Silicates of the alkaline earth metals, preferably calcium, activated by manganese, or by both manganese and lead, may be prepared as set forth in my application Serial No. 488,956 hereinbeforereferred to. Calcium silicate activated With manganese gives a series of phosphors that are excited by cathode rays to emit greenish, yellow, or orange light, depending on the amount of manganese present, and to some extent on the method of preparation. When activated with lead and excited by cathod rays, as 'disclosedin application Serial No. 396,183 of G. R. Fonda,

amounts of about .2 to 3 per cent PbO by weight,

while the mol ratio of CaO:S iO2, though not very critical, is preferably fixed at about 1:1.1 to 1:1.5.

A convenient way of making the phosphor is by direct synthesis of ingredients comprising or yielding calcium oxide, silica, lead oxide and manganese. Thus a suitable batch may consist of:

Grams Calcium carbonate (CaCOs) 1 mol or Silicic acid (SiOaxHz) to give silica (S102) 1.1 mol or 38.8 Lead oxide (PbO) 1.6

Manganese carbonate (MnCOz) having a per cent content of manganese oxide For increased red light the manganese carbonate may be increased to 3 g., and for still deeper red to 3.6 grams. The ingredients are thoroughly mixed together dry in fine state of division, and

then heated in a refractory crucible (as of porcelain, silica or Alundum) in a refractory electric muflle furnace at a temperature of about 1000 to 1250 C. in an atmosphere of steam continually supplied to one end of the furnace. The optimum time of heating depends mainly On the size of the batch, some two to three hours being satisfactory for a batch of about 90 to 100 grams according to the formula given above.

The double activated phosphor may also be produced by first preparing a calcium silicate phosphor activated with manganese alone, and then combining this with lead as a supplemental activator. For this purpose, manganese-activated calcium silicate may be prepared essentially according to the formulae and method already set forth, merely omitting the lead oxide. After cooling and sieving, the singly-activated phosphor may be mixed with the required amount of lead oxide according to the formulae, and then 4 again heated at about 1000 to 1250 C. for about an hour in an atmosphere of steam.

The manganese-activated magnesium silicate phosphor may be prepared as described in Patent 2,141,905 to G. R. Fonda et al. except that the firing takes place in an atmosphere of steam. Thus, a mixture of magnesium oxide and silica in approximately equi-molecular proportions, together with a suitable manganese compound, preferably sufficient in amount to furnish about .4 to 1.5 per cent by weight of MnO, may be fired at a temperature of about 1250" C. for about an hour or longer in an atmosphere of steam. The phosphor may also be made in proportions approximating the ortho-silicate (ZMgOzlSiOz) when fired in steam. The steam fired phosphor has a brightness'several fold that of an otherwisesimilarly prepared air fired phosphor.

Acerium-activated aluminum silicate phosphor disclosed in. Patent 2,254,956 to G. Aschermann may have its brightness doubled by firing in mixtures of steam with nitrogen or hydrogen, as against dry nitrogen or hydrogen. Thus, a suitable batch may consist of 102 parts A1203, 180 parts Si02 and 28 parts, C6203 which corresponds to about 1A12O3Z3SiO2 with about 10 per cent C6203. The phosphor may be prepared by dispersing SiO2 in a very concentrated solution of aluminum and cerium nitrates; drying; driving off nitrous gases at about 250 C.; ball milling in methyl alcohol; filtering and drying again; sieving; and firing at 1040 C. In small batches of a few grams, a firing time of 15 minutes gives good results. Thebrightness of a batch fired in steam and nitrogen was 27 (in arbitrary units) compared to a brightness of 12 when fired in dry nitrogen, and a brightness of 30 was obtained with a batch fired in steam and hydrogen compared with a brightness of 16 when fired in dry hydrogen.

The titanium-activated (with or Without manganese as supplemental activator) silicates of calcium, strontium, barium, magnesium, zinc, beryllium or cadmium may be prepared as disclosed in my application Serial No. 499,642 hereinbefore referred to, and to which reference maybe had for complete details. Thus, a batch consisting of 90 g. calcium carbonate (CaCOa) 13.6 g. calcium sulphate (CaSO4) 70.7 to 77.6 g. silicic acid SiOz) and 0.6 to 1.2 g. titanium dioxide may be compounded and fired at a temperature of about 1000-1200 C. for about two hours, in an atmosphere of steam. The brightness of the phosphor is increased a third by firing in steam compared to air firing.

The yellow-fluorescing zinc silicate may be prepared as disclosed in my application Serial No. 498,405 hereinbefore referred to. By firing in steam, a brighter phosphor may be prepared without resort to high temperatures, fusion, or quenching, or to' fluxes. Use can also be made of much larger mol proportions of zinc oxide to silica, exceeding 1:1 and as high as about 1.5:1. A preferred way of preparing the phosphor is to heat a mixture of preformed manganese silicate such as MnSiOa or MnzSiO4 with zinc oxide and silica in an atmosphere of steam at a temperature of about BOO-850 C. The manganese-silicate can be prepared by heating a thorough mixture of 1 mol MnO or MIICOa with 1 mol SiOz or more (a slight excess of SiOz being preferable) at a temperature of the order of 800 C. in an atmosphere of steam and hydrogen. Air is excluded and the heating is continued until the product is white, which may require about a half hour, de-

pending on th siz of the batch. The product is allowed to cool in the hydrogen or other reducing atmosphere.

A suitable batch may then be. made up of 30.5 g. zinc oxide (ZnO), g. silica (S102) and 2.7 g. manganese-silicate (MIISiO3)-.- The mixture is heated at a temperature of the order of,800 to 850- C., more or less. The heating is carried outin a brisk current of steam, or in a mixture of steam with neutral or reducing gas, such as pure nitrogen, carbon dioxide or hydrogen, with the exclusion of air, and may require two or three hours for a total batch of some 50 grams as in the above formula. If the phosphor is formed in steam and hydrogen, very little hydrogen should be used in order to avoid partial reduction of zinc oxide in the batch. The MnO content of the phosphor is preferably held between 1 and 5 percent, preferably about 3 per cent. The uniformity and brightness of phosphor thus produced can be improved somewhat by precalcining a mixture of ZnO and SiO2 for two or three hours at about 750 C. to form amorphous zinc silicate, and afterward mixing this product with manganese silicate (prepared as above described) and firing as described to form the phosphor. V

Fairly good results can also be obtained in a one-step firing process that combines the production of MnSiOa or the maintenance of MnO or other vehicle of bivalent manganese with the production of the matrix and the solution of MnSiOz or MnO or the like in this matrix. A suitable batch consisting of 30.5 g. zinc oxide (ZnO), 15.8 g. silica (SiOz), 1.9 g. manganese oxide (MnO) and .5 g. boric acid may be heated for three to six hours in an atmosphere of steam and hydrogen or carbon monoxide at a temperature of the order of 800 C.

In the various examples given above the firing atmosphere consists at least preponderantly of steam, and where hydrogen is admixed with the steam in the firing of phosphors such as calcium or magnesium silicate activated with manganese, thehydrogen is preferably employed in a proportion of about 20 to 40 per cent by volume with 80 to per cent steam. However, as pointed out above, in the case of the yellow zinc silicate, the hydrogen is held to a quite small amount, preferably five per cent or less. As to the amount of steam, I have obtained good results in preparing batches of the size mentioned above with a generator furnishing about 5 to 6 liters of steam per minute at 100 C. This amounts to about 20 to 24 liters per minute at the furnace temperature of about 1200 C. In a silica or alundum tube furnace of 10 sq. cm. cross-section this means about 2.4 liters of steam per sq. cm. per minute, or a flow of about 40 cc. per sq. cm. per second through the tube.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of preparing fluorescent calcium silicate activated by both manganese and lead which comprises mixing together compounds of calcium, silicon, lead and manganese selected from the group consisting of the oxides of those metals and compounds which upon heating break down to yield those oxides, the calcium and silicon compounds being present in proportions to form calcium silicate and the lead and manganese compounds being present in activating proportions, and heating the mixture at a temperature of about 1000 to 1250 C. in an atmosphere consisting at least preponderantly of steam.

2 The method of preparing fluorescent; cal-.- cium silicate activated by both manganeseyand ead w ic c i s firing at a temp at e. of about 1000- 1250 C. in an atmosphere con: sisting at least preponderantly of steam a batch consisting of calcium carbonate and, silicic acid in proportions furnishing one mol ofCaOto about 1;1 .to 1.2 mole S102, lead oxide in anamount of about .2 to 5 per cent by weight of the batch, and manganese carbonate in an amount to furnish aboutz to 4 per cent by weight of M110. 3 .The method of preparing fluorescent calcium silicate activated by both manganese and lead which comprises firing at a temperature,- of about 1000130 1250?,(3. in an atmosphere consisting of steam a batch consisting of the following ingredients in approximately the proportions stated:

Parts by weight Calcium carbonate 50 Silicic acid 38.8 Lead oxide 1.6-5

Manganese carbonate 2.4-3.6

compound of the activating metal, the several said compounds being selected from the group consisting of the respective oxides and compounds which upon heating break down to yield those oxides.

5. The method of preparing a fluorescent silicate activated by metal of the group consisting of manganese, lead, cerium and mixtures of manganese and lead which comprises firing at a temperature Within the range of about 7001300 C. in an atmosphere consisting of steam a mixture of compounds of silicon and a metal in proportions to form silicate of the said metal and an activating proportion of compound of the activating metal, the several said compounds being selected from the group consisting of the respective oxides and compounds which upon heating break down to yield those oxides.

6. The method of preparing a fluorescent silicate activated by metal of the group consisting of manganese, lead, cerium and mixtures of inanganese and lead which comprises firing at a temperature within the range of about 700-1300 C. in an atmosphere consisting of steam admixed with a non-oxidizing gas in proportions of at least about 60 per cent by volume of steam a mixture of compounds of silicon and a metal in proportions to form silicate of the said metal and an activating proportion of compound of the activating metal, the several said compounds being selected from the group consisting of the respective oxides and compounds which upon heating break down to yield those oxides.

'7. The method of preparing a fluorescent silicate activated by manganese which comprises firing at a temperature within the range of about 700-1300 C. in an atmosphere consisting at least preponderantly of steam a mixture of compounds of silicon and a metal in. proportions to form silicate of the said metal and an activating proportion of manganese compound, the several said compounds being selected from the group consisting of the respective oxides and compounds .Which upon heating break down to yield those oxides.

HERMAN C. FROELICH.

REFERENCES clTEli The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 2,306,567 Roberts Dec. 29, 1942 2,415,129 Froelich Feb. 4, 1947 2,421,207 Leverenz May 27, 1947 Certificate of Correction Patent No. 2,525,028 October 10, 1950 HERMAN C. FROELIOH It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 46, for (SiO .XH read (Sz'0 mH 0) column 6, lines 25, 39, 52 and 67, and column 7, line 4, before the word activated insert of metal of the group consisting of aim, magnesium, calcium, and aluminum; column 6, lines 31, 44 and 59, after metal insert of the first-mentioned group; same column 6, line 71, and column 7, line 8, after metal insert of the said W 2; and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of September, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commz'ssz'oner of Patents. 

4. THE METHOD OF PREPARING A FLUORESCENT SILICATE ACTIVATED BY METAL OF THE GROUP CONSISTING OF MANGANESE, LEAD, CERIUM AND MIXTURES OF MANGANESE AND LEAD WHICH COMPRISES FIRIJNG AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 700-1300*C. IN AN ATMOSPHERE CONSISTING AT LEAST PREPONDERANTLY OF STEAM A MIXTURE OF COMPOUNDS OF SILICON AND A METAL IN PROPORTIONS TO FORM SILICATE OF THE SAID METAL AND AN ACTIVATING PROPORTION OF COMPOUND OF THE ACTIVATING METAL, THE SEVERAL SAID COMPOUNDS BEING SELECTED FROM THE GROUP CONSISTING OF THE RESPECTIVE OXIDES AND COMPOUNDS WHICH UPON HEATING BREAK DOWN TO YIELD THOSE OXIDES. 